1. Field of the Invention
The present invention relates to devices and methods for comparing or correlating a state of polarization of an electromagnetic wave to a known library of such polarization states and, more particularly, to applications where the comparison of the state of polarization of an electromagnetic wave are employed, such as for target discrimination.
2. Related Art
Identifying the state of polarization of an electromagnetic wave by determining the Stokes polarization vector components of the wave is known. In particular, an electromagnetic wave, such as a spectral band of light, or of electromagnetic radiation in any spectral band, may be characterized as having four Stokes vector components (s0, s1, s2, and s3). The component s0 is proportional to the intensity of the wave. The components s1, s2, and s3 may be related to the orientation of the polarization, e.g., an ellipse and its ellipticity. In general, the orientation of polarization of a plane (planar phase front) monochromatic (single frequency) wave is elliptical. However, the ellipse may degenerate into a straight line in the case of linear polarization, and for circular polarization, the ellipse may degenerate into a circle. Electromagnetic radiation of broad bandwidth (polychromatic radiation) may be considered to comprise many signals each of which is monochromatic and which is generally elliptically polarized as described above.
An elliptically-polarized wave can be considered as the superposition of two waves of arbitrary orthogonal (perpendicular) polarization and amplitude a1 and a2 with phase difference δ. In this case, the components of a Stokes polarization vector (s0, s1, s2, and s3) may be equated to amplitude (a1 and a2) and phase difference (δ) as provided in the Table below:
TABLEs0 = a12 + a22,s1 = a12 − a22,s2 = 2a1a2 cosδ,s3 = 2a1a2 sinδ.
Accordingly, based on the equations given above, the Stokes vector is known if the parameters a1, a2, and δ are known. For further details concerning the Stokes polarization vector the reader is referred to the Principles of Optics, 3rd Edition, by M. Born and E. Wolf, Pergamon Press, Oxford, 1965, Chapter 1 which is incorporated herein by reference to the extent necessary to make and practice the present invention.
One way of measuring the Stokes vector components (s0, s1, s2, and s3) is to place two polarizers and a retarder in the optical path sequentially. Insertion of a first polarizer into an optical path gives a measure of one of the linear polarizations and a second polarizer is also inserted to give the other linear polarization. A retarder is further inserted into the optical path to retard a signal having a given sense of polarization in phase relative to a signal having another sense, where the two senses are generally orthogonal to each other. Output from the retarder is a signal containing data that can be used to calculate δ when the linear components are known. The disadvantage of this approach is that it involves moving parts, since these optical components must be placed successively in the optical path. Also, in a dynamic scene, a polarimeter using moving parts would give smeared results, since the scene could change during the times that the polarizers are being changed.
Other ways of measuring the Stokes vector components have been proposed. For example, the paper entitled “Spectroscopic Polarimetry with a Channeled Spectrum” by Kazuhiko Oka and Takayuki Kato, published in Optics Letters, Vol. 24, No. 21, Nov. 1, 1999 describes a system for spectropolarimetry which eliminates the need for inserting and removing polarizers into or out of the optical path. In particular, Oka and Kato employ a pair of birefringent retarders and an analyzer to modulate light so that the state of polarization of the light varies with frequency. The modulated light is then passed to a spectrometer or spectrum analyzer and then to a computer where, through Fourier analysis, the state of polarization of the modulated light is determined. Sabatke, et al., in Optical Engineering Vol. 41, No. 5, May 2002, describe an imaging spectropolarimeter that uses two optical retarders and a polarizer, together with a computed tomographic imaging spectrometer (CTIS), to measure a complete Stokes vector while employing no moving parts. Also, U.S. patent application Ser. No. 10/631,218, to McMillan et al, filed Jul. 25, 2003 and entitled “Coherent Radar and Ladar Polarimeter” describes a method for measuring a Stokes vector using a dual channel coherent receiver.
It is also known that light reflected or emitted from a man-made object will generally have a different polarization signature from light reflected or emitted from a natural background. For example, unpolarized light incident on a flat surface will have much of its vertical component absorbed and its horizontal component reflected.
In view of the foregoing it will be recognized that, to date, no suitable device or method of comparing a complete polarization signature of a target scene with known polarization signatures with no moving parts is available.